1. Field of the Invention
The present invention relates to a method of introducing high concentrations of dopant into semiconductor material and, in particular, to a process for doping semiconductor material through melting and recrystallization by laser radiation aided by an anti-reflective coating.
2. Description of the Related Art
Heterojunction bipolar transistors (HBT) are finding increased use in semiconductor manufacturing due to the emergence of band gap engineering to control conductance.
FIGS. 1A-1F illustrate cross-sectional views of a conventional process flow for forming an HBT.
FIG. 1A shows the starting point for the process, wherein single crystal silicon substrate 100 containing dopant of a first conductivity type is exposed to an ambient containing silane (SiH4) 102 and germine (GeH4 ) 104 gases. FIG. 1B shows the resulting deposition of silicon-germanium alloy 106 over the surface of silicon substrate 100. FIG. 1C shows ion-implantation of dopant 108 of a second conductivity type opposite the first conductivity type into the silicon-germanium alloy 106. FIG. 1D shows the subsequent formation of polysilicon layer 110 over doped silicon-germanium alloy 106. Polysilicon layer 110 is then doped to extremely high levels by ion-implantation of dopant 112 of the first conductivity type, as shown in FIG. 1E. FIG. 1F shows completion of fabrication of the HBT structure 114, wherein doped polysilicon layer 110 and underlying silicon-germanium alloy layer 106 are etched to provide polysilicon emitter 116 overlying and separated from single crystal silicon collector 118 by silicon-germanium base 120.
While FIGS. 1C and 1E depict introduction of conductivity-altering dopant into a semiconductor material (Sixe2x80x94Ge alloy 106 and polysilicon 110) utilizing ion-implantation, it is also well known to introduce dopant into semiconductor material through chemical vapor deposition (CVD) followed by furnace annealing and resulting thermal drive-in.
While satisfactory for some applications, the conventional process described above suffers from a number of disadvantages. In particular, the HBT emitter must contain an extremely high concentration of dopant, on the order of 1020-1021 atoms/cm3, in order to provide sufficient numbers of charged carriers for the device to function at high switching speeds. The maximum dopant concentrations obtainable by conventional doping techniques, such as ion-implantation or CVD, followed by thermal drive-in, are limited by physical mechanisms such as clustering. These mechanisms prevent the uniform incorporation of large numbers of dopant ions into the silicon lattice. The resulting uneven dopant distribution within the lattice adversely affects the electrical resistance and other important characteristics of the doped semiconductor material.
Therefore, there is a need in the art for a process for introducing extremely high levels of dopant uniformly within a crystalline semiconductor material.
The present invention provides a method of introducing extremely high levels of dopant uniformly within a crystalline semiconductor material. In accordance with the invention, spin-on-glass film containing high dopant concentrations is first formed over the semiconductor material. Next, an anti-reflective coating is formed over the doped spin-on-glass film. Exposure to radiation from an excimer laser heats the spin-on-glass and underlying semiconductor material to extremely high temperatures. Melting the SOG film and semiconductor material promotes diffusion of dopant, which, upon cooling and recrystallization of the semiconductor material, becomes uniformly incorporated within the semiconductor crystal lattice. The anti-reflective coating suppresses reflection of the laser beam and thereby promotes the efficient transfer of energy to melt the underlying doped spin-on-glass and semiconductor material.
In one embodiment of a method of introducing dopant into a semiconductor material, a doped layer is formed over a semiconductor material, forming an anti-reflective coating over the doped layer. A laser beam is then applied to the anti-reflective coating to melt the underlying doped layer and the semiconductor material and cause dopant from the doped layer to diffuse into the melted semiconductor material. The laser beam is then removed from the anti-reflective coating to cause the melted semiconductor material to solidify and incorporate a high concentration of diffused dopant.
The features and advantages of the present invention will be more fully understood upon consideration of the following detailed description of the invention and the accompanying drawings.